Although I'm not a dietician and not qualified to have an opinion on this, I'd say the P-butter/eggs/tuna/cereal/oatmeal diet is OK. Only thing I think should also be there, is vegetables or fruit of some kind, but the oat meal gives grain and fiber, and if you're taking supplements for vitamins, that covers some of the vegetable role. Peanut butter is fatty, but it's not a "bad fat" like animal lard, etc. Bottom line is; if it works, it works!
I eat a very limited diet as well, very similar to yours, and as you point out, it saves big $$$ on grocery bills, and it's all done in the microwave, and I don't even have the gas turned on for the stove and oven! I do eat red meat and chicken now & then, but not regularly.
Charles